It is currently more fully featured than LXDE, and significantly larger - this is because:

In the balance between features and lightness, it has gone a more towards being fully featured, even where that has meant using code which has not yet been made lean and efficient. (However it is leaner than GNOME or KDE.)

Xfce has had time to develop some packages which are not yet implemented in LXDE.

Some Xfce packages can be useful within LXDE - e.g. xfce-power-manager.

Implementations

Most distributions have Xfce in their repositories, and quite a few have Xfce versions on LiveCD. A couple of notable examples:

Xubuntu is well known, as the Xfce version of Ubuntu. Some releases have been criticized for being bloated.

CrunchBang Statler (Debian-based) offers a choice of Openbox or Xfce (or both, choosing session at login). The "Xfce" is actually a very slimmed down version, very similar to the Openbox version but using Xfwm. The developer has said that it is intended to demonstrate that a truly lightweight Xfce is possible (not his exact words).

Xfce code in LXDE

The LXDE project uses some Xfce code. It is clearly documented in the source code.

The code which is used by LXDE consists of:

ExoIconView and ExoTreeView from libexo. (These make a small fraction of libexo.)